Forests are very important to the world economy and for maintaining and preserving our ecosystem. Forest trees have a wide range of commercial uses (timber for construction, raw material for paper and pulp production, and as an energy source). The global demand for wood products (mostly for paper and pulp and firewood in developing countries) has been increasing year by year when the natural forests are in short supply. Re-forestation is the solution to meeting such increasing demand. Usually, the fast-growing, widely adapted tree species are chosen for re-forestation. Most tree-improvement programs are based on the management of genetic resources, including the selection of superior clones from existing forests, the conservation of genetic variability, partially controlled propagation and classical breeding for desired traits. In spite of the fact that it usually takes several generations to breed, this traditional breeding has been successful in getting elite trees with fast and uniform growth. However, many other traits such as disease and insect resistance, different lignin composition and content are difficult to acquire mainly due to high heterozygosity in tree species and big segregation population. Moreover, the gene(s) conferring certain phenotypes like disease resistance may not be in the gene pool at all. On the other hand, molecular breeding based on genetic transformation of tree species offers the possibility to introduce a particular phenotype without affecting the genetic background of a cultivar. Genetic transformation in Populus species and Eucalyptus species enabled some success in modification of lignin content (Tzfira et al., 1998; Robinson, 1999). The precondition of molecular breeding of forest tree species is the availability of a reliable and reproducible genetic transformation method, which in turn relies on a system of regeneration of one whole plant from a single cell.
Genus Acacia comprises about 1200 tropical and subtropical tree species. It belongs to the family Mimosaceae. Acacia mangium is a multipurpose, fast growing and nitrogen fixing elite tropical legume tree. An adult tree is up to 30-metres tall and its bole (trunk) is often straight to over half the total height. The true leaf of a seedling is a fern-like pinnate leaf. The first pinnate leaf of a seedling has 6 or 8 leaflets, and then the bipinnate leaf develops from the second pinnate leaf on a seedling. Usually when the young seedling grows up to 8-12 bipinnate leaves, the petiole is dilated into a phyllode, while the leaflets abort completely and the true leaf disappears from the young seedling. Phyllodes are flattened leaf stems which look and act like the ordinary leaves of other plants. Branchlets, phyllodes and petioles are glabrous or slightly scurfy. Phyllodes are 5-10 cm broad, 2-4 times as long as broad, dark green, and are chartaceous when dry. The phyllodes have 3-4 longitudinal main nerves which join on the dorsal margin at the base of the phyllode. Secondary nerves are fine and inconspicuous.
Flowers are in loose spikes to 10 cm long and are solitary or paired in the upper axils. Flowers are pentamerous and the calyx is 0.6-0.8 mm long with short obtuse lobes. The corolla is twice as long as the calyx. Pods are linear, glabrous, 3-5 mm broad, about 7.5 cm long when green, woody, coiled and brackish-brown when mature, and depressed between the seeds. Seeds are lustrous, black, ellipsoid, ovate or oblong, 3.5×2.5 mm, with the orangish funicle forming a fleshy aril beneath the seed (Duke, 1984). Due to A. mangium fast growth, tolerance of infertile soil and high quality fibre, it has been increasingly used for reforestation plantation and soil rehabilitation in degraded soil. Its plantation has been studied for many years in many countries or regions, especially in tropical regions and subtropical regions, such as Australian, Indonesia, Malaysia, India, Thailand, Hawaii, China and Taiwan. Many A. mangium plantations have been established in acidic soil or abandoned land or Imperata grassland, for example, in Bangladesh (Latif et al., 1995); in Sabah (Latif et al., 1995; Williams et al., 1992) and in Serdong (Majid et al., 1994; Awang, 1994) in Malaysia; Sangmelina, Cameroon, Kenya (Duguma et al., 1994); Skaerat, Thailand (Khemnark, 1994); Hawaii, USA (Cole et al., 1996); Bogor (Anwar, 1992; Wibowo et al., 1992), Paseh and Kadipaten (Widiarti and Alrasjid, 1987) in Indonesia; Bengal, India (Basu et al., 1987), etc.
Indonesia, with some of the world's largest paper and pulp mills, has been increasingly relying on plantations as the source of wood and A. mangium is the preferred choice. Asia Paper and Pulp group has two affiliate companies with a total concession of 540,000 hectares. By 1996, one company had planted 123,000 hectares of A. mangium, about 90% of all its plantation, which represented 180 million seedlings. It is estimated that by 2004, Asia Paper and Pulp group will virtually source all its wood from plantation, mainly A. mangium plantation (Bayliss, 1998a; Bayliss, 1998b).
Besides its use for paper and pulp, A. mangium timber can be used for other applications such as cement banded particle board, plywood and decorative panel manufacture (Yusoff et al., 1989; Wong et al., 1988).
A large scale of mono-plantation of vegetative propagation from a single plant faces a higher risk of disease infection. It has been found that diseases spread very fast in this kind of plantation and cause large economic losses. Many diseases devastate A. mangium: Cinnamon fungus (Phytophehera cinnamomi) infection results in collapse and death of plants; Seedling blight, defoliation and dieback due to Glomerella cingulata caused serious losses to A. mangium in nurseries. Leaf spot by Cyclindrocladium quinqueseptatum causes defoliation of seedlings and young trees; Powdery mildew (Oidium spp) severely affected A. mangium seedlings in nurseries in Thailand; Red rot diseases, caused by ganoderma sp. affects A. mangium in Malaysia; Brown root disease caused by Phellinus noxium affects A. mangium in Malaysia and the Solomon islands (Simmons, 1987; Gutteridge and Shelton, 1994). Traditional breeding has been less successful in getting disease resistance in A. mangium, mostly likely due to lack of such a gene in the natural gene pool. Molecular breeding by introducing foreign disease resistance genes becomes an important option.
Studies on A. mangium tissue culture have been limited to micropropagation (Bhaskan and Subbash, 1996; Ahmad, 1991; Galiana et al., 1991a; Galiana et al., 1991b). A combination of traditional breeding of elite trees and techniques of large-scale vegetative propagation make the large-scale plantation possible. There has been no report on regeneration or genetic transformation of A. mangium. Our invention describes the conditions for regeneration of whole plant via organogenesis and a genetic transformation system.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended List of References.